1. Field of the Invention
The invention concerns an optical fiber preform and a fabrication process for this preform.
2. Description of the Prior Art
An optical fiber is generally made from silica; its diameter is about 125 microns (.mu.m).
The silica of a multimode optical fiber is doped. The doping, for example, with germanium, is highest at the center, i.e. on the longitudinal axis, and decreases from the center towards the periphery. Accordingly, the refractive index n is the highest at the center, where its value is about 1.48, and lowest at the periphery, where its value is about 1.45, which is the index of the pure silica.
The performance of the fiber depends on the quality and the regularity of the index profile. The bandwidth is high if the index profile is regular, i.e. if the refractive index varies continuously or virtually continuously from the center towards the periphery and if the curvature of this profile is chosen correctly. Furthermore, the bandwidth depends on the longitudinal homogeneity of the index profile. Any departure from these conditions--i.e. if the index gradient has discontinuities and/or is not homogeneous in the longitudinal direction--leads to a reduction of the bandwidth. Finally, oscillations or undulations of the index profile can degrade the bandwidth to a lesser degree.
An optical fiber having a satisfactory index gradient is fabricated in the following manner: starting with a tube of pure silica having an exterior diameter between 28 mm and 36 mm and an interior diameter between 24 mm and 32 mm, for example, successive layers are deposited on the internal wall with the doping increasing from the beginning (the largest diameter) to the end (the smallest diameter). Generally there are several tens of layers.
The process employed is a modified chemical vapor deposition (MCVD) process.
To deposit the first layer a current of silicon tetrachloride SiCl.sub.4 mixed with oxygen or another oxidizing agent is passed through the interior of the silica tube and the silica tube is heated, preferably from the exterior. The heating is effected over a relatively short length; to form the layer over the greater part of the tube, the heating device is moved from the entry towards the exit of the tube. The tube is generally turned about its axis during execution of the process.
The heating causes oxidation of the silicon tetrachloride and this produces silica SiO.sub.2 and chlorine. The silica is deposited onto the inside wall of the tube and constitutes the first layer.
The subsequent layers are formed in the same manner but, for these layers, the gas additionally contains germanium tetrachloride to effect the doping. The quantity of germanium tetrachloride is varied from the second layer to the last in order to obtain the required doping--and therefore the required index--for each layer.
After deposition of the last layer, there remains an axial opening that is eliminated by heating the tube in order to shrink it. This produces a solid cylinder having an outside diameter in the order of two centimeters and a length of about one meter. This cylinder, or rod, is known as a primary preform. This primary preform is generally then covered (or "surfaced") with silica to obtain the required diameter, for example three centimeters to four centimeters.
The optical fibers proper are fabricated by drawing down such preforms. Thus a preform about one meter long and having an outside diameter of three centimeters produces in the order of 50 kilometers of optical fiber with a diameter of 125 microns.
The preform fabrication process described hereinabove is described in European patent application 301 797, for example.
It has been found that, during the fabrication of a preform by the MCVD process, a great part of the length of the preform has an index profile that differs from the profile over the remainder of the length of the preform. This lack of longitudinal homogeneity leads to a reduction of the bandwidth of the optical fibers made from this part of the preform. The lack of homogeneity of the index profile in relation to the remainder of the length manifests itself at the gas entry end, following on from a first part, starting from the entry, over which the deposit is insufficient. This first part is about 15 centimeters long. The second part, which has a varying index profile curvature different from the corresponding curvature over the remainder of the preform, extends about 10 to 15 centimeters beyond the first part. The lack of homogeneity of the doping gradient or index gradient of the second part relative to the remainder affects the bandwidth of the fiber obtained from this part or from the remainder of the preform. To summarize: about 15% of the preform is unusable and an at least--and possibly much greater--proportion of this length suffers from a lack of homogeneity of the index profile relative to the remainder of the preform, the above values being given by way of example only. Be this as it may, it has been found that the lack of homogeneity indicated above increases with the section of the layers deposited. This drawback is particularly disadvantageous at present since the aim is to obtain preforms of even greater diameter.